1. Field
The present invention relates to a thermally responsive switch having a contact switching mechanism using a thermally responsive plate such as a bimetal in a hermetic container.
2. Related Art
Thermally responsive switches of this type are disclosed in Japanese patent No. 2519530 (prior art document 1) and Japanese patent application publications JP-A-H10-144189 (prior art document 2), JP-A-2002-352685 (prior art document 3) and JP-A-2003-59379 (prior art document 4). The thermally responsive switch described in each document comprises a thermally responsive plate provided in a hermetic container comprising a metal housing and a header plate. The thermally responsive plate reverses a direction of curvature thereof at a predetermined temperature. An electrically conductive terminal pin is inserted through the header plate and hermetically fixed by an electrically insulating filler such as glass. A fixed contact is attached directly or via a support to a distal end of the terminal pin located in the hermetic container. Furthermore, the thermally responsive plate has one end fixed via a support to an inner surface of the hermetic container and the other end to which a movable contact is secured. The movable contact constitutes a switching contact with the fixed contact.
The thermally responsive switch is mounted in a closed housing of a hermetic electric compressor thereby to be used as a thermal protector for an electric motor of the compressor. In this case, windings of the motor are connected to the terminal pin or the header plate. The thermally responsive plate reverses the direction of curvature when a temperature around the thermally responsive switch becomes unusually high or when an abnormal current flows in the motor. When the temperature drops to or below a predetermined value, the contacts are re-closed such that the compressor motor is energized.
The thermally responsive switch is required to open the contacts upon every occurrence of the aforesaid abnormal condition until a refrigerating machine or air conditioner in which the compressor is built reaches an end of product's life. The thermally responsive switch needs to cut off current extremely larger than a rated current of the motor particularly when a motor is driven in a locked rotor condition or when a short occurs between motor windings. When current having such a large inductivity is cut off by the opening of contacts, arc is generated between the contacts, whereupon contact surfaces are damaged by heat due to arc. The welding of contacts occurs when the switching of contacts exceeds a guaranteed operation number. In this regard, in order that an electric path may be cut off even upon occurrence of contact welding for the purpose of preventing secondary abnormality, double safety and protective measures are taken when needed (a fusing portion of a heater described in prior art documents 1 and 2, for example).
The use of a contact containing cadmium has recently been limited for environmental reasons. For example, a silver-cadmium oxide (Ag—CdO) system contact has a small contact welding force such that the silver-cadmium oxide system contact has less wear due to arc. Accordingly, the silver-cadmium oxide system contact has been used in a large number of thermally responsive switches. Equivalent durability and current cutoff performance to those of the conventional thermally responsive switches need to be ensured by the use of an alternative contact material in the future. The current cutoff performance would be reduced by half when the silver-cadmium oxide system contact is merely replaced by a cadmiumless contact.
In order that the current cutoff performance may be improved, a structure is considered in which the size of the contacts is increased for the purpose of increasing the heat capacity, whereby occurrence of contact welding is reduced even upon occurrence of arc. Furthermore, another structure is considered in which the size of the thermal responsive plate is increased so that a force separating the contacts from each other is increased. However, when either construction is employed, the thermally responsive switch would be rendered larger in size, whereupon it would become difficult to mount the thermally responsive switch in the hermetic housing of the compressor.